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INTRODUCTION — Bowel obstruction occurs when the normal flow of intraluminal contents is interrupted. Obstruction can be functional (due to abnormal intestinal physiology) or due to a mechanical obstruction, which can be acute or chronic [1,2]. Advanced small bowel obstruction leads to bowel dilation and retention of fluid within the lumen proximal to the obstruction, while distal to the obstruction, as luminal contents pass, the bowel decompresses. If bowel dilation is excessive, or strangulation occurs, perfusion to the intestine can be compromised leading to necrosis or perforation, complications which increase the mortality associated with small bowel obstruction.

The most common causes of mechanical small bowel obstruction are postoperative adhesions and hernias. Other etiologies of small bowel obstruction include disease intrinsic to the wall of the small intestine (eg, tumors, stricture, intramural hematoma) and processes that cause intraluminal obstruction (eg, intussusception, gallstones, foreign bodies).

PATHOPHYSIOLOGY — Bowel obstruction occurs when the normal flow of intraluminal contents is interrupted. The cause of the obstruction may be external to the bowel (extrinsic), within the wall of the bowel (intrinsic), or due to a luminal defect that prevents the passage of gastrointestinal contents. Obstruction of the small intestine can be partial or complete. A type of complete obstruction, a closed-loop obstruction, occurs when the intestine is obstructed at two locations, creating a segment with no proximal or distal outlet. Closed-loop obstruction can rapidly progress to bowel strangulation. (See 'Complete obstruction and closed-loop obstruction' below.)

Normal physiology — The main function of the small intestine is to digest and absorb nutrients. Microvilli and circular folds (ie, valvulae conniventes, plicae circulares or valves of Kerkring) increase the surface area available for absorption and cause the intestinal contents to twist while flowing through the small intestine. These circular folds can be seen in radiographic studies. The small bowel is relatively free of microbes, whereas the large intestine is populated with commensal bacteria that aid digestion, synthesize a number of vitamins, and break down bilirubin.

Obstructive physiology — Obstruction leads to progressive dilation of the intestine proximal to the blockage, while distal to the blockage the bowel will decompress as luminal contents pass. Swallowed air and gas from bacterial fermentation can accumulate, adding to bowel distention. As the process continues, the bowel wall becomes edematous, normal absorptive function is lost, and fluid is sequestered into the bowel lumen [3]. There may also be transudative loss of fluid from the intestinal lumen into the peritoneal cavity. With proximal bowel obstruction, ongoing emesis leads to additional loss of fluid containing Na, K, H, and Cl, and metabolic alkalosis. These fluid losses can result in hypovolemia. Bacterial overgrowth can also occur in the proximal small bowel, which is normally nearly sterile, and emesis can become feculent.

If bowel dilation is excessive, the intramural vessels of the small intestine become compromised and perfusion to the wall of the intestine is reduced [4]. If perfusion to a segment of intestine is inadequate to meet the metabolic needs of the tissue, ischemia will occur, which will eventually lead to necrosis and perforation unless the process is interrupted [5]. Alternatively, ischemic necrosis of the bowel can be related to twisting of the bowel and/or its mesentery around an adhesive band or to lax intestinal attachments.

EPIDEMIOLOGY AND RISK FACTORS — Acute, mechanical small bowel obstruction is a common surgical emergency [1,2,6]. It is estimated that over 300,000 laparotomies per year are performed in the United States for adhesion-related obstructions [7,8]. Ischemia, which complicates 7 to 42 percent of bowel obstructions, significantly increases mortality associated with bowel obstruction [5].

The small bowel is involved in about 80 percent of cases of mechanical intestinal obstruction [9,10]. The incidence is similar for males and females. In one Polish study of adult patients, the average age of patients with acute obstruction was 64 years, women comprised 60 percent of the group, and the small bowel was affected in 76 percent [9].

Risk factors — Factors that increase the risk for the most common etiologies of small bowel obstruction are given in the table (table 1). (See 'Specific etiologies' below.)

The most important risk factors include:

●Prior abdominal or pelvic surgery

●Abdominal wall or groin hernia

●Intestinal inflammation

●History of, or increased risk for neoplasm

●Prior irradiation

●History of foreign body ingestion

The most important risk factor for mechanical small bowel obstruction in the United States is prior abdominal surgery causing postoperative adhesions. In a systematic review, the incidence of postoperative bowel obstruction due to any cause was 9.4 percent; the incidence related to adhesive disease was 2.4 percent [11]. Patients with a history of prior abdominal or pelvic surgery, and particularly colorectal surgery, appendectomy, gynecologic surgery, prior adhesiolysis, and resection of malignancy are prone to adhesive small bowel obstruction [4,11-13]. The risk of early postoperative bowel obstruction due to adhesions, which is defined as bowel obstruction occurring during the same hospitalization as the index operation, is increased after exploration for trauma. The incidence was reported at 3.9 percent in a review of 571 patients, a rate that was increased fourfold in the presence of traumatic gastrointestinal perforation [14]. (See "Traumatic gastrointestinal injury in the adult patient".)

For patients with a history of prior bowel obstruction, whether managed medically or surgically, the likelihood of recurrent obstruction increases with an increasing number of episodes [2,15]. Postoperative adhesions may also be responsible for chronic abdominal pain. In a systematic review, five studies evaluated the incidence of chronic abdominal pain following surgery. Chronic abdominal pain was attributable to postoperative adhesions in 34 to 67 percent of patients [11]. In women, chronic pelvic pain and infertility can also result from adhesions [16]. (See "Causes of chronic pelvic pain in women" and "Causes of female infertility", section on 'Fallopian tube abnormalities/pelvic adhesions'.)

Adhesive small bowel obstruction can occur in the absence of prior abdominal surgery. In one study of 34 patients without prior history of abdominal surgery, who underwent exploratory surgery for small bowel obstruction, 29 (85 percent) had adhesions as the cause of the obstruction [17]. Adhesions may have developed in these patients as a result of intraabdominal inflammation (eg, diverticulitis, Crohn disease). Alternatively, other pathologies, such as hernia or volvulus, can lead to small bowel obstruction by causing extrinsic compression of the small bowel.

Diseases intrinsic to the wall of the small intestine (eg, tumor, stricture, intramural hematoma) can cause small bowel obstruction by encroaching upon the lumen of the bowel because of edema, infiltration of the bowel wall, or from progressive stricture formation.

CLINICAL PRESENTATIONS — Patients with bowel obstruction can present with an abrupt onset of abdominal pain, nausea, vomiting, and abdominal distention, or with intermittent, acute symptoms that resolve only to recur again. Some patients with chronic, partial obstruction may develop superimposed symptoms and signs of acute bowel obstruction [1,6,16,18-29].

The history should seek to identify risk factors for bowel obstruction, which will provide clues to the potential etiology of suspected bowel obstruction or suggest an alternative diagnosis [30,31]. In addition, prescription or nonprescription medications (including recreational drugs) that may impact bowel function should be noted (table 2). (See 'Risk factors' above and 'Specific etiologies' below and 'Differential diagnosis' below.)

The frequency of these symptoms is variable and depends upon the etiology and location of obstruction (proximal versus distal), and degree of obstruction (partial versus complete). One review of 300 patientsreported abdominal pain in 92 percent of patients, and vomiting in 82 percent [19]. In a prospective study of 150 patients, the absence of the passage of flatus (90 percent) or stool (81 percent) was the most common presenting symptom [5]. In a study of patients with adhesive small bowel obstruction, the presenting symptoms were cramping abdominal pain in 68 percent, vomiting in 77 percent, absence of passage of flatus and/or feces in 52 percent, and constant pain in 12 percent [24].

Abdominal pain associated with small bowel obstruction is frequently described as periumbilical and cramping with paroxysms of pain occurring every four or five minutes [26]. A progression from cramping to more focal and constant pain may indicate peritoneal irritation related to complications (ischemia, bowel necrosis). A sudden onset of severe pain may suggest acute intestinal perforation. (See "Overview of gastrointestinal tract perforation".)

With proximal small bowel obstruction (duodenum, proximal jejunum), nausea and vomiting can be relatively severe, and patients with proximal small bowel obstruction typically cease taking in food or liquids orally.

Physical examination — The physical examination should evaluate the patient overall for systemic signs related to the bowel obstruction. A hallmark of small bowel obstruction is dehydration, which manifests as tachycardia, orthostatic hypotension, and reduced urine output, and if severe, dry mucus membranes. Fever may be associated with infection (eg, abscess) or other complications of obstruction (ischemia, necrosis), and although fever suggests infection, its absence does not rule it out, particularly in older or immunocompromised patients. Hematochezia may be a sign of tumor, ischemia, or inflammatory mucosal injury, or intussusception.

In multiple retrospective reviews, abdominal distension was the most frequent physical finding on clinical examination occurring in 56 to 65 percent of patients [5,19,24]. Although nausea and vomiting may be less severe in patients with distal small bowel obstruction compared with proximal obstruction, abdominal distention is greater because the more proximal bowel acts as a reservoir. Swallowed air and gas from bacterial fermentation can also accumulate, adding to the abdominal distention. It is important to remember, however, in patients with a closed-loop obstruction, abdominal distention can be minimal. (See 'Pathophysiology' above.)

Acute mechanical bowel obstruction is characterized by high-pitched “tinkling” sounds associated with the pain. With significant bowel distention, bowel sounds may become muffled, and as the bowel progressively distends, bowel sounds can become hypoactive.

Distention of the bowel results in hyperresonance or tympany to percussion throughout the abdomen. However, fluid-filled loops will result in dullness. If percussion over the liver is tympanitic rather than dull, it may be indicative of free intraabdominal air. Tenderness to light percussion suggests peritonitis.

Abdominal palpation may identify any abdominal wall or groin hernias, or abnormal masses, which, in the setting of small bowel obstruction, may indicate an abscess, volvulus, or tumor as the source of obstruction. Digital rectal examination should be performed to identify fecal impaction or rectal mass as the source of obstruction. Gross or occult blood may be related to intestinal tumor, ischemia, inflammatory mucosal injury, or intussusception.

Laboratory studies — The typical laboratory evaluation for patients who present with significant abdominal pain includes a complete blood count with differential and electrolytes including blood urea nitrogen and creatinine. Although routine laboratory studies are not specific for a diagnosis of small bowel obstruction, these studies help assess the presence and severity of hypovolemia, leukocytosis, and metabolic abnormalities (hyponatremia, hypokalemia). Leukocytosis with leftward shift may indicate the presence of complications. Anemia may point to a specific etiology (eg, Crohn’s disease, tumor, Meckel’s diverticulum).

In patients who present with systemic signs (eg, fever, tachycardia, hypotension, altered mental status), additional laboratory investigation should include arterial blood gas (ABG), serum lactate, and blood cultures. Metabolic alkalosis can result from severe vomiting, but metabolic (lactic) acidosis can also occur if the bowel becomes ischemic, or if hypovolemia is severe enough to cause hypoperfusion of other organs [32]. Although there is no reliable clinical or laboratory marker for ischemia, elevated serum lactate is sensitive, but not specific, for ischemia in patients with small bowel obstruction (sensitivity 90 to 100 percent, specificity 42 to 87 percent) [27,28].

Chronic, partial obstruction — Chronic small bowel obstruction occurs in a fixed segment of bowel and the obstruction is, by definition, partial. The most common causes of chronic, partial small bowel obstruction are chronic stricture from Crohn’s disease, adhesions from prior surgery, slowly-growing tumors, and stricture related to prior bowel resection or irradiation.

Patients usually present with chronic postprandial abdominal discomfort and variable nausea. Abdominal distention and tympany may be present, but usually without any fluid or electrolyte derangements. When a patient with chronic, partial small bowel obstruction becomes completely obstructed, the clinical presentation becomes indistinguishable from acute obstruction as described above.

Recurrent obstruction — The patient who presents with recurrent, intermittent obstruction, typically due to adhesions, is distinguished from the patient with a chronic, partial small bowel obstruction. Recurrent obstruction due to adhesions can occur in a fixed segment of bowel or differing segments of bowel. Those that occur at the same site due to a focal band adhesion are more likely to respond to surgery compared with those that occur at varying locations within the abdomen due to diffuse adhesions, for which surgery is likely to increase the risk of future obstructions.

During an episode of obstruction, symptoms are identical to those of patients with acute small bowel obstruction described above, but symptoms resolve and the patient may report postobstructive diarrhea. In the period between obstructive episodes, the patient is usually asymptomatic with a normal abdominal exam.

For patients with a history of prior bowel obstruction, whether managed medically or surgically, the likelihood of recurrent obstruction increases with an increasing number of episodes, and the asymptomatic time period between episodes decreases [2]. After three prior episodes, the likelihood of recurrent obstruction is >80 percent [15].

DIAGNOSIS — Although mechanical small bowel obstruction may be suspected (or obvious) based upon risk factors, symptoms, and physical exam findings consistent with obstruction, abdominal imaging is usually needed to confirm the diagnosis, identify the location of obstruction, judge whether the obstruction is partial or complete, identify complications related to obstruction (ischemia, necrosis, perforation) and determine the potential etiology, all of which will help determine the urgency and nature of further treatment (conservative, endoscopy, surgery) [33,34]. (See "Overview of management of mechanical small bowel obstruction in adults".)

Confirming the diagnosis — Multiple imaging modalities are available to confirm a suspected diagnosis of small bowel obstruction, but plain radiography and computed tomography of the abdomen are the most practical and useful. For most patients, we obtain plain radiographs to quickly confirm a diagnosis of bowel obstruction and, provided the films do not have findings that indicate the need for immediate intervention, we use computed tomography (CT) of the abdomen to further characterize the nature, severity, and potential etiologies of the obstruction.

Abdominal CT has largely replaced fluoroscopic studies for this purpose, but these and other studies, such as ultrasound, endoscopy, and magnetic resonance enterography, may be useful in certain patient populations. (See 'When to obtain other studies' below.)

Plain radiography — For most patients, we suggest plain radiographs to quickly confirm a diagnosis of bowel obstruction because it is widely available, inexpensive, and may demonstrate findings that indicate the immediate need for urgent decompression (eg, sigmoid volvulus) or surgical intervention (eg, pneumoperitoneum, cecal or midgut volvulus) [35]. Plain radiography also assesses the lungs for evidence of aspiration in those who have been vomiting, and can easily be repeated to follow the patient’s progress. (See "Overview of management of mechanical small bowel obstruction in adults", section on 'Serial monitoring'.)

The basic radiologic examination should include an upright chest film and upright and supine abdominal films (image 1 and image 2). If the patient cannot be placed into an upright position, a lateral decubitus abdominal film can show free air and/or air-fluid levels.

Findings on plain radiography consistent with small bowel obstruction include the following:

●Dilated loops of bowel with air-fluid levels

•In the supine position, the air-fluid interface is parallel to the x-ray plate, and the entire width of air and fluid-filled loops of bowel will be visible (image 3). This allows an estimation of the amount of distention.

•In an upright (or lateral) position, the air-fluid interface is perpendicular to the film and is evident as an air-fluid level (image 4). Multiple air-fluid levels with distended loops of small bowel are seen in small bowel obstruction.

●Proximal bowel dilation with distal bowel collapse – Small bowel obstruction can be diagnosed if the more proximal small bowel is dilated more than 2.5 cm (outer wall to outer wall) and the more distal small bowel is not dilated [5,36]. The stomach may also be dilated. The presence of air-fluid levels differing more than 5 mm from each other within the same loop of small bowel on upright films supports a diagnosis of mechanical small bowel obstruction [37].

●Gasless abdomen – A gasless abdomen may be due to complete filling of loops of bowel with sequestered fluid. The severity of the bowel obstruction may be underestimated. A string of beads (or pearls) sign may be seen in predominantly fluid-filled small bowel loops on upright or lateral films, as small amounts of intraluminal gas collect along the superior bowel wall separated by the valvulae conniventes [36].

The site or cause of obstruction is usually not apparent on plain films since a transition point between the dilated proximal and nondilated distal small bowel often cannot be established with certainty. For a diagnosis of small bowel obstruction using plain radiographs, the sensitivity, specificity, and accuracy are 79 to 83 percent, 67 to 83 percent, and 64 to 82 percent, respectively [37-39].

Although plain abdominal films have a reasonable sensitivity for the detection of high-grade small bowel obstruction, they are less useful differentiating small from large bowel obstruction, and differentiating partial obstruction from ileus [38,40].

Abdominal CT — Multidetector CT of the abdomen is more useful than plain radiographs for identifying the specific site (ie, transition point) and severity of obstruction (partial versus complete) [35]; determining the etiology by identifying hernias, masses [41,42], or inflammatory changes; and for identifying complications (ischemia, necrosis, perforation) [36]. Plain films can be equivocal in 20 to 30 percent of patients and are "normal, nonspecific, or misleading" in 10 to 20 percent of patients [5,43]. Thus, some have argued that because abdominal CT is more effective for identifying patients who will need intervention, abdominal CT should be performed initially instead of plain films. However, we generally obtain plain abdominal films (flat and upright/left lateral decubitus) prior to proceeding to abdominal CT scan, since radiographs are readily available, less expensive, expose the patient to less radiation, and may obviate the need for abdominal CT in some patients. (See 'Plain radiography' above.)

One study compared the efficacy of plain radiography and CT scan in 32 patients presenting with a clinical suspicion for bowel obstruction [39]. The sensitivity and specificity of CT scanning was 93 and 100 percent, respectively, compared with 50 and 75 percent for plain radiography. The level of obstruction was correctly predicted in 93 percent on CT scan compared with only 60 percent for plain films. CT was also superior for determining the cause of the obstruction (87 versus 7 percent). In another study, the negative predictive value of abdominal CT for excluding strangulation was 95 percent [44]. For high-grade small bowel obstruction, the sensitivity, specificity, and accuracy of CT scan are reported to be 90 to 94 percent, 96 percent, and 95 percent, respectively [45-50]. For low-grade obstruction, the accuracy of CT is reduced [51]. It is important to note that the sensitivity and specificity of CT scan for a diagnosis of bowel obstruction increases with an increasing number of slices. In a systematic review and metaanalysis, the sensitivities and specificities for various slice widths were as follows [18]:

Similar to the findings on plain abdominal radiography, a diagnosis of bowel obstruction on abdominal CT can be made by the findings of dilated proximal bowel with distal collapsed bowel, and air-fluid levels (image 5 and image 6) [5]. (See 'Plain radiography' above.)

●“Whirl sign” – rotation of small bowel mesentery, suggesting a twist or a volvulus

●“Venous Cut-off Sign” – venous flow to a loop of small bowel that is “cut off” suggests thrombosis

Identifying the transition point between dilated and nondilated bowel, although not required to make the diagnosis of obstruction, may establish the site and cause of small bowel obstruction [36]. Reconstruction in multiple planes allows the tortuous small bowel to be followed in the search for a transition point, which may be a sharply defined point, as with band adhesions, or a longer segment, as seen with matted adhesions or radiation enteritis. However, the location of obstruction as identified on CT only correlates with the intraoperative locations in about 60 to 70 percent of patients [56,57]. In addition, the presence of a transition point on abdominal CT does not appear to accurately predict the need for immediate or delayed operative intervention, and thus should not be used as a major initial criterion influencing a decision to operate [14,56]. (See "Overview of management of mechanical small bowel obstruction in adults", section on 'Indications for surgical exploration'.)

The administration of oral and intravenous contrast optimizes the information provided by abdominal CT scanning (image 7A-B). However, for those who cannot tolerate oral contrast, retained intraluminal fluid within dilated bowel loops usually provides adequate enhancement when evaluating patients for ischemic complications. A lack of bowel wall enhancement, an early sign of ischemia, may be easier to identify in the absence of oral contrast agents. (See 'Bowel ischemia and perforation' below.)

Complete obstruction and closed-loop obstruction — The history should determine whether the patient is continuing to pass any gas or stool from the rectum. Cessation of passage of stool or flatus indicates a complete obstruction, which is more likely to be associated with complications (ischemia, necrosis, perforation). However, it is important to remember that passage of flatus or feces can continue for 12 to 24 hours after the onset of symptoms as the more distal bowel decompresses. The absence of air or fluid in the distal small bowel or colon on plain abdominal radiographs or CT scan supports a diagnosis of complete obstruction.

A special type of complete obstruction, a closed-loop obstruction, may be more difficult to identify on radiologic studies. A closed-loop obstruction occurs when a segment of intestine, usually small bowel, is obstructed in two locations, creating a segment with no proximal or distal outlet. Only a short segment of intestine may be distended because of minimal abdominal distention. Closed-loop obstruction can rapidly lead to complications (ischemia, necrosis, perforation); thus, early identification and treatment are important to restore perfusion to the affected segment of bowel. In many cases abdominal exploration will be needed to make a definitive diagnosis [58].

On imaging studies, a closed-loop obstruction often appears as a distended, fluid-filled, sometimes C-shaped or U-shaped bowel segment (image 8) with prominent mesenteric vessels converging on a point of torsion (CT whirl sign) or incarceration [52]. Other signs include a triangular loop, the beak sign, and the presence of two collapsed bowel loops adjacent to the obstruction site [53].

Bowel ischemia and perforation — Patients with complications (ischemia, necrosis, perforation) related to bowel obstruction are typically very ill in appearance and may have systemic signs of toxicity and laboratory features that suggest that complications related to bowel obstruction have occurred, such as lactic acidosis, and leukocytosis.

Bowel ischemia leading to intestinal necrosis and perforation almost always occurs in the setting of complete obstruction. An exception is a Richter's hernia, a condition in which only a portion of the intestinal wall protrudes through a hernia defect and which can lead to ischemia and perforation without complete obstruction of the lumen (figure 1).

A diagnosis of perforation relies on the finding of extraluminal air in diagnostic imaging studies.

●Pneumoperitoneum is a sign of perforation of the intraabdominal gastrointestinal tract (small bowel, transverse colon, sigmoid colon) and may be detected as:

•Free air under the diaphragm on upright chest or upright abdominal radiograph (image 9)

•Free air over the spleen or liver on lateral abdominal film or abdominal CT

•Free air as a “football sign” on supine abdominal film or abdominal CT

●Air in the retroperitoneum may indicate perforation of the duodenum or retroperitoneal portions of the colon.

•Psoas sign on supine abdominal film

•Air adjacent to the second portion of the duodenum on plain abdominal film or abdominal CT

Abdominal CT is more sensitive for detecting extraluminal air, particularly air in the retroperitoneum, and in some patients, free air demonstrated on CT scan is not apparent on plain radiographs.

Abdominal CT is also better than plain radiographs for detecting signs of bowel ischemia. Advanced bowel ischemia can generally be recognized on CT scan (image 10); however, lesser degrees of ischemia are more difficult to confidently diagnose [5]. Findings associated with small bowel ischemia on abdominal CT are listed below [59-63]. However, none of these signs are highly sensitive or specific [59,60]. The presence of a combination of these findings increases the reliability of diagnosing ischemia [64,65].

When to obtain other studies — Other studies, such as ultrasound, magnetic resonance enterography, and gastrointestinal contrast studies can also be used to evaluate the patient with suspected bowel obstruction. These adjunctive studies can be time-consuming and invasive, and following plain radiographs we recommend abdominal CT over any of these studies for difficult-to-diagnose bowel obstruction, which is more likely to be a problem in patients who present with chronic symptoms [66]. (See 'Abdominal CT' above.)

Abdominal ultrasonography — Abdominal ultrasonography may be useful for the diagnosis of small bowel obstruction in selected patients. Ultrasound is limited by poor visualization of gas-filled structures [39], but is increasingly used in the emergency department to evaluate abdominal pain [18], to assess for occult hernias, which may be the site of incarcerated small bowel, and in patients with contraindications to CT scanning, such as those with contrast allergies [35], pregnant patients, and critically ill patients for whom the study can be performed at the bedside [67].

Ultrasound is more sensitive and specific than plain films for the diagnosis of small bowel obstruction [18,68,69], but not as helpful for determining the location, cause, and potential complications of bowel obstruction. In a study that compared the efficacy of plain radiography, ultrasound, and CT scan in 32 patients presenting with clinical suspicion of intestinal obstruction [39], the sensitivity and specificity of ultrasound were 75 percent each, compared with CT scanning at 93 and 100 percent, respectively. The level of obstruction was correctly predicted in 70 percent of patients using ultrasound, but in 93 percent of patients on CT scan. Ultrasound was also inferior to CT for determining the cause of the obstruction (23 versus 87 percent). In another study, the positive predictive value of an akinetic dilated loop on ultrasound for strangulation was 73 percent [70].

Magnetic resonance enterography — Magnetic resonance (MR) enterography is an increasingly attractive option for the assessment of small bowel obstruction [36]. Multiplanar MR enterography can be used in the same way as multidetector computed tomography to look for evidence of a transition point and features indicative of complications. However, the increased time for image acquisition and the need for repeated breath-holds to obtain high-quality images limit the general applicability of MR enterography in patients with acute small bowel obstruction.

MR enterography is most useful in the setting of low-grade bowel obstruction, provided the patient can tolerate the procedure. MR enterography may therefore be most suitable for diagnosing bowel obstruction associated with chronic small bowel conditions such as Crohn’s disease, a population of younger patients for whom reducing the accumulated dose of ionizing radiation is highly desirable [36,71]. In a study of 28 patients, MR (HASTE MR, Siemens) had a higher sensitivity and specificity than helical CT for the cause of obstruction [72].

Small bowel contrast studies — Although small bowel follow-through series and enteroclysis can confirm the diagnosis of bowel obstruction and can determine if an obstruction is partial or complete, they are inferior to abdominal CT for detecting closed-loop obstruction or ischemia, and only rarely point to the etiology of the obstruction. Thus, these studies have a limited role in the initial diagnosis of small bowel obstruction [36,73]. Small bowel follow-through is contraindicated if there are any signs of strangulation.

Fluoroscopic findings consistent with small bowel obstruction are dilated loops of proximal small bowel opacified with contrast material, and a change in the diameter of the small bowel at the transition zone. Minimal or no contrast material opacifying the small bowel loops distal to the transition zone indicates high-grade obstruction.

The transition zone at the site of small bowel obstruction can be missed using small bowel follow-through (or abdominal CT) because water-soluble contrast agents become diluted as they pass through dilated fluid-filled bowel loops. Consequently, the degree of opacification may not be sufficient to identify the transition point at the site of obstruction. The transition zone is, however, readily identified with enteroclysis.

Enteroclysis is a procedure in which the duodenum is intubated with a nasojejunal tube, and a large volume of air and contrast (barium and methylcellulose) are instilled directly into the small intestine while repeatedly imaging over time using fluoroscopy. The volume challenge caused by methylcellulose administration accentuates the effect of low-grade obstruction. For patients with chronic or recurrent small bowel obstruction, enteroclysis distends the small bowel sufficiently to identify areas of stenosis [36,74]. However, enteroclysis is not appropriate for patients with acute obstruction. The prolonged transit time of contrast material through obstructed bowel means that follow-through radiographs require several hours to obtain, which can delay the diagnosis. In addition, patients with acute small bowel obstruction tolerate oral contrast material poorly, and it is preferable not to have large quantities of barium in the small bowel lumen if surgery is a possibility.

Contrast enema — Contrast enema can be helpful to diagnose small bowel obstruction in patients who have undergone proctocolectomy with an ileoanal J pouch reconstruction (eg, ulcerative colitis) or patients with a subtotal colectomy and ileorectal anastomosis (eg, Crohn’s disease, ulcerative colitis, cancer). Water soluble contrast is preferred over barium if bowel perforation is a concern.

DIFFERENTIAL DIAGNOSIS — Nausea and vomiting may be a manifestation of a myriad of disorders (table 3). Although bowel obstruction is uncommon relative to all the possible causes of nausea and vomiting and abdominal pain, it is important to make the diagnosis expediently because of the potential for complications. For many medical conditions, vomiting is more likely to precede the onset of significant abdominal pain, whereas pain often precedes vomiting when associated with an acute surgical etiology. (See "Evaluation of the adult with abdominal pain" and "Causes of abdominal pain in adults".)

Bowel dilation due to mechanical obstruction must be differentiated from nonobstructive intestinal motility disorders such as adynamic (paralytic) ileus and intestinal pseudo-obstruction (table 4). Either disorder can be related to electrolyte derangement, major surgery, trauma, intestinal ischemia, or peritonitis from other causes. Patients can develop the same symptoms as obstruction, but these disorders can be distinguished from mechanical bowel obstruction by history and abdominal imaging.

Adynamic (paralytic) ileus — Paralytic ileus occurs to some degree after almost all open abdominal operations and can also be caused by peritonitis, trauma, intestinal ischemia, and medications (eg, opiates, anticholinergics). It is exacerbated by electrolyte disorders, particularly hypokalemia. As the intestine becomes distended, the patient experiences many of the same symptoms as mechanical obstruction. However, on radiologic examination there is air in the colon and rectum, and on abdominal computed tomography (CT) or small bowel series there is no demonstrable mechanical obstruction [75,76].

In differentiating early postoperative ileus from postoperative adhesive disease, it is useful to note that nearly all patients with early postoperative bowel obstruction have an initial of return of bowel function and oral intake, which is then followed by nausea, vomiting, abdominal pain, and distention, whereas patients with adynamic ileus do not experience return of bowel function [77]. (See "Postoperative ileus".)

Pseudoobstruction — Intestinal pseudoobstruction is a chronic condition characterized by symptoms of recurrent abdominal distention that may be associated with nausea, vomiting, and diarrhea. The colon is generally affected more than the small intestine. No mechanical cause can be demonstrated, and the patient frequently has a history of several previous operations for bowel obstruction during which no cause for obstruction could be found. (See "Chronic intestinal pseudo-obstruction".)

Large bowel obstruction — The clinical presentation of large bowel obstruction depends upon the location and etiology of the obstruction. Tumor is the most common cause of large bowel obstruction, followed by adhesive disease and volvulus.

Diseases affecting the proximal colon, such as cecal volvulus or adhesions, are more likely to be confused with acute small bowel obstruction compared with distal disease. In a study of patients with colonic volvulus, the most frequent symptoms were abdominal pain (58 percent) and obstipation (55 percent) [25]. Most patients with cecal volvulus presented with abdominal pain, which occurred in 89 percent of patients. The interval at which cramping pain occurs is typically longer with colonic obstruction compared with small bowel obstruction, and occurs lower in the abdomen between the umbilicus and pubic tubercle.

In the large bowel, adenocarcinoma of the colon and rectum is the predominant malignancy causing obstruction. Colonic obstruction complicates 10 to 20 percent of colon cancers. Because tumors are slow growing and often located more distally in the colon, symptoms are chronic and progressive, and rarely confused with acute small bowel obstruction. Tumors that cause colon obstruction commonly cause “apple-core” lesions, which are readily demonstrated on CT scan. In addition to identifying a mass as a cause of large bowel obstruction, the abdominal CT scan may demonstrate a synchronous lesion, enlarged lymph nodes, or metastases. (See "Clinical presentation, diagnosis, and staging of colorectal cancer".)

SPECIFIC ETIOLOGIES — Once a diagnosis of small bowel obstruction has been established, it is important to try to determine the specific etiology (table 1) responsible for the obstruction. The patient’s age or medical history often suggests the possible etiology. (See 'Epidemiology and risk factors' above and 'Clinical presentations' above.)

In the United States and Western Europe, the most common cause of mechanical small bowel obstruction is intraperitoneal adhesions followed by tumors and complicated hernias [5,9]. Less frequent causes of obstruction include Crohn’s disease (3 to 7 percent) [6,23], gallstones (2 percent), volvulus (4 to 15 percent) [20,78,79], and intussusception (4 to 8 percent) [78,80].

Patients who present with clinical manifestations consistent with a small bowel obstruction but who do not have a history of prior abdominal surgery, and have none of the other common risk factors for bowel obstruction, should be presumed to have small bowel tumor until proven otherwise. (See "Epidemiology, clinical features, and types of small bowel neoplasms".)

Unique clinical and diagnostic features associated with the more common etiologies of small bowel obstruction are briefly reviewed below and discussed more fully in linked topic reviews. The general clinical presentation and diagnosis of bowel obstruction are discussed above. (See 'Clinical presentations' above and 'Diagnosis' above.)

Adhesive bowel disease — Adhesions are the most common etiology for small bowel obstruction, with up to 70 percent of cases of bowel obstruction in developed countries attributable to adhesions within the abdomen or pelvis [36]. Approximately 80 percent of patients with adhesive small bowel obstruction have a history of prior intraabdominal surgery; the remainder have prior peritonitis or no precipitating cause for their adhesions [36].

Peritoneal adhesive bands are the most frequent etiology of bowel obstruction following abdominal or pelvic surgery. Although intraabdominal adhesions form in more than 90 percent of patients after open abdominal surgery [81], few patients require surgery to manage adhesive bowel obstruction. In large reviews, between 7 and 17 percent of patients will require admission for small bowel obstruction related to adhesions, but of these, only 2 to 5 percent will need adhesiolysis [13,82,83].

A specific preoperative diagnosis of adhesions as a cause of bowel obstruction is difficult to confirm [53]. However, because adhesions commonly involve the omentum or mesenteric fat, signs that indicate abnormalities of the normal architecture may indicate that adhesions are responsible for bowel obstruction. Signs include a “fat-bridging sign,” which is a cord-like structure containing mesenteric fat that can bridge across the peritoneum; twisting of the mesentery (whirl signs); and tethering of the omentum. Adhesions are a frequent cause of closed-loop obstruction [84], which, in addition to the identification of abnormal bands on imaging, may appear as a sac-like clustering of intestine indicating that the intestine has herniated into an enclosed space [84].

Tumor — Tumors, predominantly metastatic malignant tumors, are the second most common cause of small bowel obstruction, accounting for about 20 percent of cases [85].

Primary tumor — Primary tumors of the small or large bowel may be responsible for symptoms and signs of small bowel obstruction.

Intraluminal small bowel neoplasms such as carcinoid, small bowel carcinoma, and lymphoma can cause small bowel obstruction due to luminal narrowing or intussusception. In one review of 17 patients, the most frequent primary small bowel tumors as an etiology for small bowel obstruction were gastrointestinal stromal tumors (GIST) (36 percent), lymphomas (24 percent), and adenocarcinomas (18 percent). Most tumors (65 percent) were located in the ileum [86]. In patients with symptoms and imaging consistent with small bowel obstruction but who did not have a history of prior abdominal surgery, and have none of the other common risk factors for bowel obstruction, additional studies may be needed to rule out small bowel tumor if the lesion is not apparent on initial imaging. (See "Epidemiology, clinical features, and types of small bowel neoplasms" and "Diagnosis and staging of small bowel neoplasms".)

Metastatic disease — Metastatic disease is the most frequent neoplastic cause of small bowel obstruction. In general, small bowel obstruction caused by metastases are frequently preceded by a period of partial small bowel obstruction, although acute obstruction can sometimes be due to twisting of the bowel around a metastatic tumor deposit leading to small bowel volvulus. In a patient with a prior history of surgery, a small bowel obstruction due to metastases cannot be differentiated from a small bowel obstruction due to adhesive disease.

Tumors with a propensity to cause widespread peritoneal metastases include colonic, ovarian, pancreatic, and gastric neoplasms [36]. Small bowel obstruction has been described in as many as 28 percent of patients with colorectal carcinoma and 42 percent of women with ovarian carcinoma [87]. Multiple serosal small bowel metastases can form confluent soft-tissue masses that surround the bowel. Obstruction occurs by extrinsic compression of the small bowel lumen or tethering of bowel loops by these serosal deposits.

Complicated hernia — Hernias are the third leading cause of intestinal obstruction, accounting for about 10 percent of all cases, and incarcerated hernias are the leading cause of complications (ischemia, necrosis, perforation) related to bowel obstruction [1,5,79,88].

External hernias occur at sites of muscular or ligamentous weakness in the abdominal wall. Abdominal wall and inguinal hernias are most commonly represented; femoral, obturator, and parastomal hernias can also be complicated by small bowel obstruction.

Internal hernias cause 0.6 to 6.0 percent of small bowel obstructions and occur through acquired or congenital defects in the mesentery (table 5) [89,90]. Acquired internal hernias can be due to adhesions, or from artificial mesenteric openings created during the course of an operation, such as during small bowel Roux-en-y (eg, Roux-en-y gastric bypass, pancreaticoduodenectomy) or ileal conduit procedures [84,91,92].

Some patients with abdominal wall or groin hernia may present with intermittent obstructive symptoms if their hernia remains reducible; however, incarcerated hernias that cause bowel obstruction typically present acutely. Abdominal wall, inguinal, and femoral hernias can usually be detected on clinical examination. Small incisional hernias and hernias in obese individuals may not be clinically evident, but can usually be identified on CT scan. Typical findings of hernias on abdominal CT are discussed elsewhere. (See "Classification, clinical features, and diagnosis of inguinal and femoral hernias in adults", section on 'Diagnosis'.)

Traumatic intramural hematoma — A history of blunt abdominal trauma (recent or remote), in the absence of other risk factors for bowel obstruction, should suggest a diagnosis of traumatic intramural hematoma as the etiology of acute mechanical bowel obstruction. Late fibrosis of a segment of bowel previously affected can lead to chronic symptoms of obstruction due to gradual narrowing of the intestinal lumen [93,94].

The duodenum is the most frequently involved segment of the bowel because it is fixed in the retroperitoneum and easily compressed between the abdominal wall and the vertebral column. A common cause is injury from a seatbelt. Other intestinal sites of hematoma causing acute mechanical obstruction have also been described [95-97].

Intestinal stricture — Intestinal stricture as an etiology of bowel obstruction can be due to a number of disorders, including Crohn's disease, certain drugs such as enteric-coated potassium chloride solutions and NSAIDs, radiation therapy, and mesenteric ischemia [98]. Strictures can also occur at the site of prior gastrointestinal anastomoses.

Patients with inflammatory bowel disease (eg, Crohn’s disease) can present with small bowel obstruction due to adhesions or small bowel stricture. These patients usually present with chronic, intermittent symptoms from partial rather than complete bowel obstruction. Small bowel obstruction as the source for symptoms may be overlooked because of the long-standing nature of disease symptoms, and it may be difficult to distinguish a Crohn’s exacerbation from small bowel obstruction. Additional imaging studies may be needed. (See "Clinical manifestations, diagnosis and prognosis of Crohn disease in adults", section on 'Diagnosis' and 'When to obtain other studies' above.)

Small bowel stricture can also result from an episode of mesenteric ischemia. Because the ileocolic artery is the last branch of the superior mesenteric artery, the usual site of ischemic stricture in the small bowel is the distal ileum [99]. (See "Overview of intestinal ischemia in adults".)

Volvulus — Volvulus refers to twisting of a segment of the intestinal tract around a fixed point, often leading to acute mechanical bowel obstruction. The most common sites of volvulus are the cecum and sigmoid colon [101,102]. Small bowel volvulus is less common in adults, and it is usually due to congenital intestinal anomalies (primary small bowel volvulus), but may be related to a prior abdominal procedure, for which intestinal anatomy has been altered, or tumors (secondary small bowel volvulus) [103,104]. (See "Intestinal malrotation in adults", section on 'Acute clinical presentation'.)

Gallstones or foreign body — Rarely, acute mechanical small bowel obstruction can be caused by intraluminal material. The site of obstruction is usually at the ileocecal valve, where the lumen of the bowel is smallest.

●Gallstone ileus occurs when a large gallstone erodes into the small bowel via a biliary-enteric fistula. In addition to typical symptoms of small bowel obstruction and imaging findings consistent with small bowel obstruction, other imaging findings of gallstone ileus include biliary air (pneumobilia) and the finding of an aberrantly located large gallstone, which is often impacted at the ileocecal valve (image 15 and image 16). (See "Gallstone ileus".)

●Gastrointestinal bezoars, which are composed of ingested material that is not digested within the gastrointestinal tract, can obstruct the bowel lumen and may be related to a high-fiber diet, improperly chewed food, hair ingestion, and medications (table 6) [105]. Abdominal radiographs, abdominal ultrasound, or CT scan may show the bezoar as an intraluminal mass or a filling defect. (See "Gastric bezoars".)

●Heavy intestinal parasitic infestation with Ascaris lumbricoides can lead to acute mechanical intestinal obstruction, and in endemic areas, ascariasis causes up to one-third of all bowel obstructions, typically in children, but adults can also be affected [106-109]. Patients will have typical symptoms of acute mechanical small bowel obstruction, and emesis may contain worms. An abdominal mass may be appreciated on physical examination. (See "Ascariasis".)

Intussusception — Intestinal intussusception is rare in adults, accounting for 1 to 5 percent of mechanical bowel obstructions [110,111]. In adults, intussusception is typically due to pathologic lead point within the bowel, which is malignant in over half of cases [89,110]. The lead point is pulled forward by normal peristalsis, telescoping or prolapsing the affected segment of bowel (intussusceptum) into another segment of bowel (intussuscipiens) (picture 1) [110,112]. An increased incidence of intussusception has been reported in patients with acquired immune deficiency syndrome (AIDS) [90,112]. This is due to the high incidence of infectious and neoplastic conditions of the bowel in AIDS patients, such as lymphoid hyperplasia, Kaposi's sarcoma, and non-Hodgkin's lymphoma. (See "Epidemiology, clinical features, and types of small bowel neoplasms".)

Intussusception can be classified by etiology (benign lesion [eg, polyps, Meckel’s diverticulum], malignant lesion, or idiopathic) or by location as entero-enteric, which is limited to the small bowel; ileo-colic with prolapse of the terminal ileum into the ascending colon; and colo-colic, which is limited to the large bowel.

In adults, intermittent abdominal pain is the most common presentation; however, patients can also present with symptoms consistent with intermittent partial bowel obstruction with nausea, vomiting, melena, weight loss, fever, and constipation [112]. Plain abdominal films may show the typical features of distal small bowel obstruction. The diagnosis is often made on abdominal CT [112,113]. The distended loop of bowel appears thickened because it represents two layers of bowel [36]. A “target sign” may be seen on the sagittal view of the abdominal CT (image 17), while on axial or coronal view, the intussusception will appear as a sausage-shaped mass.

Other unusual etiologies

●Splenosis, which is autotransplanted splenic tissue usually as a result of traumatic splenic rupture, can cause bowel obstruction due to extrinsic compression (image 18), but intussusception has also been reported [114-116].

●Superior mesenteric artery syndrome is an unusual cause of proximal small bowel obstruction. The syndrome is characterized by compression of the third portion of the duodenum due to narrowing of the space between the superior mesenteric artery and aorta, and is primarily attributed to loss of the intervening mesenteric fat pad. (See "Superior mesenteric artery syndrome".)

●Congenital anomalies of the gastrointestinal tract can cause obstructive symptoms. Although rare in adults, malrotation and annular pancreas can cause proximal small bowel obstruction. (See 'Volvulus' above and "Intestinal malrotation in adults".)

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●Bowel obstruction occurs when the normal flow of intraluminal contents is interrupted, which leads to bowel dilation and sequestration of fluid within the lumen of the intestine proximal to the blockage, while distal to the blockage, as luminal contents pass, the bowel decompresses. (See 'Introduction' above and 'Pathophysiology' above.)

●Compromised blood flow to the intestinal tissue due to excessive bowel dilation or strangulation can lead to complications (ischemia, necrosis, perforation), which significantly increase mortality associated with bowel obstruction. (See 'Introduction' above and 'Pathophysiology' above.)

●The most common cause of mechanical small bowel obstruction is postoperative adhesions from prior abdominal or pelvic surgery. Adhesions cause extrinsic compression of the bowel. Patients who have undergone appendectomy, gynecologic surgery, prior adhesiolysis, exploration for abdominal trauma, and prior resection for malignancy are particularly prone to adhesive small bowel obstruction. Adhesive small bowel obstruction can occur in the absence of prior surgery due to intestinal inflammation (Crohn’s disease, diverticular disease). Other etiologies that cause extrinsic compression of the intestine are hernia and volvulus. (See 'Epidemiology and risk factors' above and 'Adhesive bowel disease' above and 'Complicated hernia' above and 'Volvulus' above.)

●Bowel obstruction can also be due to disease intrinsic to the wall of the intestine (eg, tumor, stricture, intramural hematoma) or processes that cause intraluminal obstruction (eg, intussusception, gallstones, foreign body). Patients who present with symptoms consistent with a small bowel obstruction but who do not have a history of prior abdominal surgery, and have no other obvious risk factors (eg, hernia, inflammatory bowel disease) for bowel obstruction, should be presumed to have small bowel tumor until proven otherwise. (See 'Epidemiology and risk factors' above and 'Tumor' above and 'Intestinal stricture' above and 'Traumatic intramural hematoma' above and 'Gallstones or foreign body' above and 'Intussusception' above.)

●The clinical presentation of mechanical small bowel obstruction depends upon the site and etiology of obstruction. Patients can present with acute symptoms of nausea, vomiting and abdominal pain, intermittent obstructive symptoms (asymptomatic periods between episodes), or with chronic postprandial discomfort and abdominal distention. Vomiting can be severe in patients with proximal small bowel obstruction, while in patients with distal small bowel obstruction, abdominal distension may be more prominent. (See 'Clinical presentations' above.)

●In patients with acute small bowel obstruction, routine laboratory studies help assess the presence and severity of hypovolemia and electrolyte abnormalities, and may indicate the possibility of complications (eg, leukocytosis, metabolic acidosis). Although there are no reliable laboratory markers for bowel ischemia, elevated serum lactate is sensitive, but not specific. Laboratory studies in patients with chronic bowel obstruction are usually normal. (See 'Laboratory studies' above.)

●A presumptive diagnosis of acute mechanical small bowel obstruction can be made by history and physical examination in many patients, particularly those with a history of prior abdominal surgery, or an obviously strangulated hernia. However, abdominal imaging is generally needed to confirm a diagnosis of mechanical bowel obstruction, identify the location of obstruction, judge whether the obstruction is partial or complete, identify complications related to obstruction (ischemia, necrosis, perforation) and determine the potential etiology, all of which will help determine the urgency and nature of further treatment (conservative, endoscopy, surgery). (See 'Diagnosis' above and 'Differential diagnosis' above.)

●For most patients, we suggest obtaining plain abdominal films to confirm a suspected diagnosis of mechanical small bowel obstruction. Findings on plain film that indicate that small bowel obstruction is present include dilated loops of bowel with air-fluid levels, proximal bowel dilation with distal bowel collapse, or a gasless abdomen. For many patients, no further radiologic tests will be needed because the films may demonstrate findings that indicate the immediate need for urgent decompression (eg, sigmoid volvulus) or surgical intervention (eg, pneumoperitoneum, cecal or midgut volvulus). (See 'Plain radiography' above.)

●Abdominal computed tomography (CT) is the imaging modality of choice for identifying the specific site (ie, transition point, small versus large bowel) and severity of obstruction (partial versus complete, or closed-loop obstruction); determining the etiology by identifying hernias, masses, or inflammatory changes; and for identifying complications (ischemia, necrosis, perforation). (See 'Abdominal CT' above and 'Complete obstruction and closed-loop obstruction' above and 'Bowel ischemia and perforation' above.)